Students undertaking summative and formative assessment are very much aware of the imperative of the outcome and often find themselves in a stressful situation with a high level of required concentration. Usability testing of assessment tool interfaces is hindered by the ability to sufficiently replicate the exam environment or to disrupt, watch or monitor a student while they are undertaking an exam or rely on their memories to reproduce their concerns at a later stage. This research demonstrates how a set of heuristics is adapted and redefined to enable an iterative approach to the improvement of a computer aided online assessment tool. The revised set of heuristics offers a tool for future developers to assist in the development of online assessment interfaces.